1. Field
The present disclosure relates to a method for nondestructively testing, and more particularly, to a method of testing adhesively bonded structures.
2. Discussion of the Related Technology
Keeping pace with the recent trend toward the use of increasing number of structures fabricated with high strength steels, there have been proposed a variety of methods for inspecting a welding condition and monitoring a fatigue-caused crack with respect to a bonded part of the steel structures. These methods are nondestructive in view of the fact that the monitoring task for a bonded condition should be conducted without destroying the structures. Examples of major testing methods developed thus far includes a radiography, an ultrasonic flaw detecting method, a magnetic powder flaw detecting method, a dye infiltrating method, an eddy current method and so forth.
However, such methods are problematic in terms of the following aspects. Among others, the radiography makes use of a radiation harmful to the human body and therefore may injure not only the health of an operator in the testing process but also the health of a person who comes close to the monitored structures with a residual radiation. Furthermore, the radiography exhibits a reduced testing speed, costs a great deal in conducting the test and has no ability to perform a real time test while the structures are in operation.
The ultrasonic flaw detecting method is said to be safer and more economical than the radiography. But the ultrasonic flaw detecting method makes it difficult to analyze the signals indicative of the ultrasonically detected results and may suffer from a problem in processing the signals due to the presence of noises. Additionally, the ultrasonic flaw detecting method requires a signal reception and analysis process, thus making it impossible to carry out the test on a real time basis.
In case of the magnetic powder flaw detecting method, the material that can be monitored by the same is confined to a conductive one. Further, no accurate test result can be derived because a high voltage current for forming magnetic fields adversely affects a test device. In addition, the magnetic powder flaw detecting method has no ability to carry out the test on a real time basis. Just like the magnetic powder flaw detecting method, the eddy current flaw detecting method is applicable only to a conductive material and has a disadvantage in that the test results are sensitively changed depending on the surface condition of the bonded part.
Also known is an acoustic inspection method that can monitor the condition of an overall structure on a real time basis and can be applicable to a wide range of materials. However, the acoustic inspection method shows different test results depending on the sensitivity of sensors employed and is heavily affected by the shape of a structure, thereby encountering a difficulty in accurately detecting the location of a flaw.
In addition to the above, the exemplary nondestructive test techniques including the radiography, the ultrasonic flaw detecting method, the magnetic powder flaw detecting method, the dye infiltrating method, the eddy current method and the like are restrictively used in detecting an internal defect, a surface flaw and a damage of those steel structures fabricated by welding joints. For this reason, the exemplary test techniques are unsuitable for use with adhesively bonded joints or composite structures made of newly developed materials. Moreover, the exemplary test techniques require the use of costly equipments and lack an ability to predict the remaining life span of a structure because they are dedicated to testing the current structure condition.
In consideration of the drawbacks posed in the exemplary test methods, extensive researches have been made for a technique of testing the load delivery capability and the integrity of adhesively bonded joints on a real time basis. As a result, there have been developed a variety of methods for diagnosing the integrity of a structure on a real time basis. U.S. Pat. No. 5,245,293 discloses a method for real-time testing the bonded part integrity of adhesively bonded structures. In this patent, a bonded condition is detected by allowing an electric current to flow through object members bonded together by an adhesive agent and then measuring the change in resistance or capacitance thereof. However, the technology disclosed is adapted to monitor the bonded condition not by the quantitative values of the resistance or capacitance but by the change in such values. Inasmuch as the capacitance of the bonded part varies sensitively with the ambient temperature, the humidity, the length of conductive lines, the floating capacitance or the like, the technology disclosed has a drawback in that it has no ability to accurately judge the bonded condition.
Korean Patent Application No. 10-2003-0066640 teaches a method for testing the bonded part integrity of adhesively bonded structures through the use of a piezoelectric characteristic of an adhesive. In this technology, use is made of the piezoelectric characteristic that the adhesive agent emits electric charges as it is pressed. Namely, the integrity of the adhesively bonded structures is tested by connecting conductive lines to the objects bonded together and monitoring the quantity of electric charges emitted through the conductive lines.
FIG. 1 is a graph showing the correlation between a density of electric charges and a fatigue life at the time when an adhesive agent receives a specific kind of fatigue load. In FIG. 1, the density of electric charges is obtained by dividing the quantity of electric charges by the bonding area of the objects bonded with an adhesive agent, and the fatigue life represents a value measured by applying the specific kind of fatigue load to the adhesive agent on a cycle-by-cycle basis.
Under a fixed load, the stress acting on an adhesive agent is inversely proportional to the area of a bonded surface and the density of electric charges is proportional to the stress, which means that the density of electric charges is increased in proportion to the decrease of the bonded area. Accordingly, an increase in the density of electric charges means a decrease in the bonded area which in turn represents an increase in the separated area. Thus, if physical properties (a piezoelectric constant, a tensile strength, a compression strength, a sheer strength and the like) are measured for varying kinds of adhesive agents and if the kinds of loads applied (a tensile load, a compression load and a sheer load) are determined, after which the correlation between an electric charge density and a fatigue life is set in advance, it becomes possible to evaluate the integrity and the remaining life of bonded structures by measuring the quantity of the electric charges generated from a bonded part at an arbitrary time. Further, since the quantity of the electric charges is measured over the entire bonded parts, the integrity test can be conducted for the bonded parts as a whole. The correlation between the stress developed in the bonded structures and the electric charge density is disclosed by the inventor of the subject patent application (2003), “Piezoelectric Monitoring of the reliability of adhesive joints”, Journal of Adhesion Science Technology, Vol. 17, No. 6, pp. 777-796.
A exemplary technique that uses, for the purpose of testing bonded part integrity, an epoxy-based adhesive agent which is one of typical adhesive agents and exhibits an extremely weak piezoelectric characteristic. According to this technique, the epoxy-based adhesive agent, which is disposed between target objects to bond them together, emits electric charges by its piezoelectric characteristic in the event that a pressure is applied thereto. Thus, if the bonded objects are electrically connected to one another, it is possible to measure the quantity of the electric charges flowing therebetween. As shown in Table 1 below, however, the typical epoxy-based adhesive agent has an extremely low piezoelectric constant. Therefore, the epoxy-based adhesive agent has a shortcoming in that a relatively small quantity of electric charges is measured in the process of testing bonded part integrity of bonded structures, thus reducing the test sensitivity. In order to easily detect the relatively small quantity of electric charges, it is unavoidable to employ such a means as an amplifier.
TABLE 1Piezoelectric StressPiezoelectric StrainAdhesive AgentConstant (pC/N)Constant (10−3 C/m2)PZT(Pb(Zn—Ti)O3)12018PVDF3016PVC13Nylon 110.30.5Rubber0.10.0001~0.001Epoxy (Rubber Toughened)0.0290.027Epoxy (Without Rubber)0.0190.057
The foregoing discussion in this section is to provide general background information, and does not constitute an admission of prior art.